1. Field of the Invention
This invention relates to auto-focus cameras, and more particularly to an apparatus for controlling the stepwise focusing movement of a lens barrel.
2. Description of the Prior Art
In the conventional camera's lens barrel driving apparatus for controlling the stepwise focusing movement of the photographic lens, a rotor having a moving coil printed thereon is arranged in a magnetic field and a current supply of prescribed direction (hereinafter called a "normal" current supply) to the coil is recycled to axially move the photographic lens stepwise. In more detail, for one pulse of normal current supply to the coil, the rotor rocks in one direction, driving a ratchet of the barrel to advance one tooth. As the pulse then fades, the rotor and the ratchet mechanism are brought back to their initial positions by a common return spring. Such a procedure is repeated a number of times depending on the object distance until the photographic lens is focused on the object.
Since the driving torque of the rotor is limited by the capacity of the electrical source or battery, because, in such a conventional apparatus, not only the mass of the lens barrel but also the bias force of the return spring is loaded on the rotor, a much-desired increase in the working speed of the rotor could not be achieved. Also since the time necessary to reset the rotor and the ratchet mechanism to the initial position depends on the magnitude of the driving force of the aforesaid return spring, there was automatically determined a limit on the shortening of the time necessary to advance the ratchet by one tooth and then to reset it to the initial position, or the period of one cycle of operation.
From such reasons, the prior known apparatus of this kind had a problem that the focusing of the photographic lens barrel takes so long a time that as the time lag from the actuation of a camera release to the start of a shutter operation is long, when shooting a moving object, the image in the frame of film is caused to differ from the finder image. The shorter the shooting time lag, the easier the management of the camera can be said to be. In this respect, there has been a demand for speeding up the focusing of the photographic lens.